Normal human serum (NHS) is able to kill T. b. brucei, but not T. b. rhodesiense and T. b. gambiense. The lytic factor was identified as being apolipoprotein L-I (apoL1) that forms pore in endosomal membranes of the parasite. This protein is associated with HDL particles that are efficiently taken up by the parasite through specific binding to a haptoglobin-hemoglobin surface receptor, due to the simultaneous presence of haptoglobin-related protein (Hpr) acting as a ligand in these particles.
The T. brucei subspecies rhodesiense and gambiense resist this lytic activity and can infect humans, causing sleeping sickness. T. b. rhodesiense resistance to lysis involves interaction of the Serum Resistance-Associated (SRA) protein with the C-terminal helix of apoL1.